Aromatic hydrocarbons, particularly benzene, toluene, ethylbenzene and xylenes, are important commodity chemicals in the petrochemical industry. Currently, aromatics are most frequently produced from petroleum-based feedstocks by a variety of processes, including catalytic reforming and catalytic cracking. However, as the world supplies of petroleum feedstocks decrease, there is a growing need to find alternative sources of aromatic hydrocarbons.
One possible alternative source of aromatic hydrocarbons is methane, which is the major constituent of natural gas and biogas. World reserves of natural gas are constantly being upgraded and more natural gas is currently being discovered than oil. Because of the problems associated with transportation of large volumes of natural gas, most of the natural gas produced along with oil, particularly at remote places, is flared and wasted. Hence the conversion of alkanes contained in natural gas directly to higher hydrocarbons, such as aromatics, is an attractive method of upgrading natural gas, providing the attendant technical difficulties can be overcome.
A large majority of the processes currently proposed for converting methane to liquid hydrocarbons involve initial conversion of the methane to synthesis gas, a blend of H2 and CO. However, production of synthesis gas is capital and energy intensive and hence routes that do not require synthesis gas generation are preferred.
A number of alternative processes have been proposed for directly converting methane to higher hydrocarbons. One such process involves catalytic oxidative coupling of methane to olefins followed by the catalytic conversion of the olefins to liquid hydrocarbons, including aromatic hydrocarbons. For example, U.S. Pat. No. 5,336,825 discloses a two-step process for the oxidative conversion of methane to gasoline range hydrocarbons comprising aromatic hydrocarbons. In the first step, methane is converted to ethylene and minor amounts of C3 and C4 olefins in the presence of free oxygen using a rare earth metal promoted alkaline earth metal oxide catalyst at a temperature between 500° C. and 1000° C. The ethylene and higher olefins formed in the first step are then converted to gasoline range liquid hydrocarbons over an acidic solid catalyst containing a high silica pentasil zeolite.
However, oxidative coupling methods suffer from the problems that they involve highly exothermic and potentially hazardous methane combustion reactions and they generate large quantities of environmentally sensitive carbon oxides.
A potentially attractive route for upgrading methane directly into higher hydrocarbons, particularly ethylene, benzene and naphthalene, is dehydroaromatization or reductive coupling. This process typically involves contacting the methane with a catalyst comprising a metal, such as rhenium, tungsten or molybdenum, supported on a zeolite, such as ZSM-5, at high temperature, such as 600° C. to 1000° C. Frequently, the catalytically active species of the metal is the zero valent elemental form or a carbide or oxycarbide.
For example, U.S. Pat. No. 4,727,206 discloses a process for producing liquids rich in aromatic hydrocarbons by contacting methane at a temperature between 600° C. and 800° C. in the absence of oxygen with a catalyst composition comprising an aluminosilicate having a silica to alumina molar ratio of at least 5:1, said aluminosilicate being loaded with (i) gallium or a compound thereof and (ii) a metal or a compound thereof from Group VIIB of the Periodic Table.
In addition, U.S. Pat. No. 5,026,937 discloses a process for the aromatization of methane which comprises the steps of passing a feed stream, which comprises over 0.5 mole % hydrogen and 50 mole % methane, into a reaction zone having at least one bed of solid catalyst comprising ZSM-5, gallium and phosphorus-containing alumina at conversion conditions which include a temperature of 550° C. to 750° C., a pressure less than 10 atmospheres absolute (1000 kPaa) and a gas hourly space velocity of 400 to 7,500 hr−1.
Moreover, U.S. Pat. Nos. 6,239,057 and 6,426,442 disclose a process for producing higher carbon number hydrocarbons, e.g., benzene, from low carbon number hydrocarbons, such as methane, by contacting the latter with a catalyst comprising a porous support, such as ZSM-5, which has dispersed thereon rhenium and a promoter metal such as iron, cobalt, vanadium, manganese, molybdenum, tungsten or a mixture thereof. After impregnation of the support with the rhenium and promoter metal, the catalyst is activated by treatment with hydrogen and/or methane at a temperature of about 100° C. to about 800° C. for a time of about 0.5 hr. to about 100 hr. The addition of CO or CO2 to the methane feed is said to increase the yield of benzene and the stability of the catalyst.
However, the successful application of reductive coupling to produce aromatics on a commercial scale requires the solution of a number of serious technical challenges. For example, the reductive coupling process is both endothermic and thermodynamically limited. Thus the cooling effect caused by the reaction lowers the reaction temperature sufficiently to greatly reduce the reaction rate and total thermodynamic conversion if significant make-up heat is not provided to the process.
Moreover, the process tends to produce carbon and other non-volatile materials that accumulate on the catalyst resulting in reduced activity and potentially undesirable selectivity shifts. In addition, at the high temperatures involved in the process, the active metal species (MoCx, WCx, etc) on the catalyst may migrate, agglomerate or change phase, again resulting in undesirable declines in conversion and selectivity. The catalyst is therefore subjected to frequent oxidative regeneration to remove the carbon and other non-volatile materials that have accumulated on the catalyst and possibly to redistribute the active metal species. However, depending on the composition of the catalyst, oxidative regeneration may have certain unwanted ancillary effects. For example, the metal on the catalyst may be converted from a catalytically active elemental or carburized state to a less active oxidized state. Also, following regeneration, the catalyst may exhibit enhanced activity for coke deposition and related hydrogen generation. Thus the present invention seeks to provide an improved method for regenerating a dehydroaromatization catalyst and subsequently tailoring its activity for use in the conversion of methane to aromatic hydrocarbons.
U.S. Patent Application Publication No 2003/0083535 discloses a process for aromatization of a methane-containing feed, in which a dehydroaromatization catalyst is circulated between a reactor system and a regenerator system, where the catalyst is contacted with different regeneration gases, including O2, H2, and H2O, at different times to regenerate different portions of catalyst. The percentage of catalyst contacting each regeneration gas is controlled to maintain the reactor system and regeneration system under a heat balance regime. The reactor system includes a fluidized bed of catalyst in a riser reactor, and the regeneration system includes a second fluidized bed of catalyst maintained in a bubbling bed reactor. After passage through the regeneration system, the hot regenerated catalyst is returned to the reactor system by way of a transfer system, which may include a reduction vessel for increasing the activity of the regenerated catalyst by contacting the catalyst in a fluidized bed with a reducing gas stream including hydrogen and/or methane.
However, processes that use the catalyst regeneration step to supply reaction heat, such as that described in the '535 publication, suffer from the problem that the catalyst needs to be heated well above the target reaction temperature in the regeneration process, which leads to accelerated catalyst degradation and hence reduced catalyst life. In addition, the entire catalyst inventory must be circulated between the reactor system and the regenerator system and hence is subject to the extreme conditions in the regenerator. Moreover, to maintain heat balance, the process requires a high selectivity to coke rather than to the desired aromatic products.